Table 6. r² Values for Bond Length vs pK_a Plots for the Five Candidate Tautomer/Conformer Forms^a.

	tautomer/conf	(im)N–H (i)	N=C (ii)	C–N(sec) (iii)	C–N(pr) (iv)	(pr)N–H (v)	C–N(pr) (iv′)
full set	A	0.324	0.925	0.843	0.018	0.072
	B	0.064	0.650	0.649	0.083	0.010
	C		0.257		0.149	0.014	0.237
	D	0.062	0.172	0.848	0.798	0.677
	E	0.005	0.469	0.563	0.380	0.445
w/o outliers	A	0.360	0.977	0.823	0.077	0.042
	B	0.037	0.644	0.524	0.088	0.302
	C		0.408		0.501	0.077	0.244
	D	0.064	0.328	0.852	0.887	0.813
	E	0.002	0.469	0.563	0.380	0.445

^aAs the bond order changes throughout structures A–E; (im) is an imino-type nitrogen; (sec) denotes a secondary amino nitrogen; (pr) denotes a primary amino nitrogen. C lacks a single-bonded secondary nitrogen and has an additional primary amino-type, denoted (iv′). The q² and RMSEE values are also listed in Tables S11 to S15 of the Supporting Information.